The present invention relates to a magnetic resonance imaging method for the acquisition of two 3-dimensional datasets, where spatial encoding by three mutually orthogonal magnetic field gradients is performed such that signal is readout under a readout-gradient in one spatial direction k1, and spatial encoding in the other two spatial directions k2, k3 is performed by applying phase encoding gradients in the other two spatial directions prior to signal acquisition, and data acquisition is performed in a sequential manner such that at each acquisition step signal is acquired under said readout-gradient, but with different combinations of the two phase encoding gradients, the method comprising a turbo spin echo train being used to acquire one of the 3-dimensional datasets, whereby                a single excitation RF pulse is being followed by multiple refocusing RF pulses,        the time interval between an excitation RF pulse and a first refocusing RF pulse is half of the time interval of two adjacent refocusing RF pulses,        one k-space readout is acquired in between two refocusing RF pulses,        phase encoding gradients between each two consecutive refocusing RF pulses are varied to acquire different phase encoding steps,        a minimum of two echo trains with a waiting time between successive echo trains is being used for the encoding of the 3-dimensional dataset.        
A method of this type is known from Klaus Scheffler and Jürgen Hennig Magn. Reson. Med 45:720-723 (2001) (=Reference [3]).
The present invention relates generally to magnetic resonance imaging (=MRI) technology. It specifically relates to data acquisition methods for MRI.
Magnetic resonance imaging is a relative new technology compared with computed tomography (=CT) and the first MR Image was published in 1973 by P. C. Lauterbur in “Image Formation by Induced Local Interactions: Examples of Employing Nuclear Magnetic Resonance”, Nature 242, 190491. It is primarily a medical imaging technique which most commonly used in radiology to visualize the structure and function of the body. It could provide detailed Images of the body in any plane. MRI provides much greater contrast between the different soft tissues of the body than CT does, making it especially useful in neurological, cardiovascular, and oncological imaging. It uses a powerful magnetic field to align the nuclear magnetization of hydrogen atoms in water in the body. Radio frequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
An MRI system typically establishes a homogenous magnetic field, generally along a central axis of a subject undergoing an MRI procedure. This homogenous main magnetic field affects the magnetic properties of the subject to be imaged by aligning the nuclear spins, in atoms and molecules forming the body tissue. If the orientation of the nuclear spins is perturbed out of alignment, the nuclei attempt to realign their spins with the field. Perturbation of the orientation of the nuclear spins is typically caused by application of radio frequency (RF) pulses tuned to the Larmor frequency of the material of interest. During the realignment process, the nuclei precess about the direction of the main magnetic field and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or around the subject.
Magnetic resonance imaging employs temporally and spatially variable magnetic fields to encode position by affecting the local Larmor frequency of spins. Gradient coils typically used for that purpose generate spatial encoding magnetic fields (=SEMs) which are superimposed on the main magnetic field. This allows to choose the localization of the image slices and also to provide phase encoding and frequency encoding. This encoding permits identification of the origin of resonance signals during image reconstruction. The image quality and resolution depends significantly on the strength and how the applied encoding fields can be controlled. Control of the gradient coils is generally performed in accordance with pre-established protocols or sequences at events, called pulse sequences, permitting different types of contrast mechanisms to be imaged.
In 3D turbo spin echo (aka TSE, FSE, RARE, SPACE) sequence (see References [1, 2]), long repetition time TR is usually used in T2 weighted imaging and proton-density weighted imaging. To reduce image blurring caused by signal decay in a series of acquired echoes in each repetition time TR, relative short echo train length is used, which results in low time utilization ratio: Only a small portion of the repetition time TR is used to acquire data, and the rest of the time is purely spent waiting for signal recovery (referred to as “waiting time” in following sections).
Non-selective refocusing RF pulses with variable flip angles were used to achieve short echo spacing, and enable 3D TSE imaging with relative longer echo trains. However, the typical time utilization ratio was still below 30% in typical examinations.
The application of multiple slab acquisition in 3D TSE imaging was employed to improve the time utilization by applying interleaved multi-slab excitation in each TR. However, the combined 3D image from multi-slab acquisition is prone to slab boundary artifacts, caused by spatially varying RF excitation profiles.
The present invention presents a way to substantially overcome one or more of the disadvantages in the existing methods of prior art discussed above.
A specific object of the present invention is to propose a data acquisition method in MRI, where the data are acquired by repeating the timing of a series of RF pulses and varying the spatial phase encoding steps of the readouts. The time interval between two consecutive applications of the same series of RF pulses is referred to as the repetition time TR.